1. Field of the Invention
This invention relates to apparatus and methods for use above a separation unit flash zone to reduce entrainment of droplets of liquid entrained in a vaporous stream as it leaves the flash zone.
2. Discussion of the Prior Art
Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Higher efficiencies and lower costs are achieved if the crude oil separation is accomplished in two steps: first, the total crude oil is fractionated at essentially atmospheric pressure; second, a bottoms stream of high boiling hydrocarbons, which typically have an initial boiling point below about 800.degree. F. (427.degree. C.), is fed from the atmospheric distillation unit to a second distillation unit operating at a vacuum pressure. The bottoms stream from the atmospheric distillation is also known as topped crude.
The vacuum distillation unit typically separates the atmospheric unit bottoms into gas oil vapors catagorized as light gas oil having a boiling point between about 420.degree. and about 610.degree. F. (216.degree.-320.degree. C.), heavy gas oil having a boiling point between about 610.degree. and about 800.degree. F. (320.degree.-427.degree. C.), vacuum gas oil having a boiling range between about 800.degree. and about 1050.degree. F. (427.degree.-566.degree. C.), and vacuum reduced crude having a boiling point of about 1050.degree. C..sup.+ (566.degree. C.). The vacuum reduced crude is also known as residuum and leaves the vacuum distillation unit as a liquid bottoms stream. Additional information concerning distillation is available in Petroleum Refining Technology and Economics, Gary, J. H. and Handwerk, G. E., pp. 31-51, Marcel Dekker, Inc. (1975).
The vacuum pressure allows the distillation unit to separate the atmospheric unit bottoms into fractions at lower temperatures than if separation were at atmospheric pressure. The high temperatures necessary to vaporize the atmospheric unit bottoms at atmospheric pressure cause thermocracking to occur, with loss in C.sub.5.sup.+ yield due to formation of gas, discoloration of the product, and equipment fouling due to coke formation.
In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons because of entrainment.
Entrainment is undesirable because the entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, which can poison downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking, to which portions of the lighter hydrocarbons are typically fed. Most downstream catalytic processes employ fluid beds or fixed beds or catalyst. For example, a gas oil product, from a vacuum or atmospheric distillation column, may subsequently feed a fluid catalytic cracking unit. If there are metals contained in the feed to a fixed bed hydroconversion process, such as soluble or organometallic compounds, the bed will generally become increasingly plugged with metals as they deposit on the catalyst. These metals deposit themselves in the interstitial space between the catalyst particles, causing the pressure drop to increase. For either a fluid bed or fixed bed catalytic process, the depositing metals decrease the activity of the catalyst. Therefore, it is desirable to minimize metals, especially nickel and vanadium, which may adversely affect catalyst selectivity and life.
The metals enter lighter hydrocarbons, such as gas oil, by two routes: (1) by vaporization, because the organometallic compounds have a finite vapor pressure, although their vapor pressure is extremely low and by far the greatest amount of the metallic compounds are in the very heaviest fraction of the bottoms; and (2) by liquid entrained with the gas oil vapors. The elimination of entrainment can only eliminate the metals present in the gas oil via the second route. However, because of the low volatility of the metal compounds, reduction of entrainment should significantly reduce metals content in the lighter hydrocarbons and thus improve performance of downstream catalytic units.
In vacuum distillation, bottoms separated from crude oil by an atmospheric distillation unit are fed to a flash zone in the lower portion of the vacuum distillation unit. To reduce entrainment of residiuum from the flash zone, along with the lighter hydrocarbons, such as gas oil, a demister or wire mesh pad is frequently installed at some point between the flash zone and a gas oil draw-off. However, the demister or wire mesh pad is not completely satisfactory for a number of reasons:
(1) entrainment in many cases is not found to be significantly reduced;
(2) the pads have a tendency to plug with heavy oil and other material; and
(3) the pads have a tendency to corrode, with holes resulting from the corrosion.
Methods other than the demister pads have been tried in the past to reduce the entrainment of residuum into the gas oil, but these methods have met with only limited success. Employing a conventional bubble-cap tray above the flash zone causes the vapor to pass through liquid on the bubble-cap tray, thereby allowing vapor to re-entrain liquid droplets. These re-entrained droplets may contain less of the higher boiling components; however, their presence in the vapor stream is deleterious to good fractionation and downstream processing. In addition, the bubble-cap tray exhibits a pressure drop, thus increasing the flash zone pressure required to drive the vapor through the bubble-cap tray. Increased pressure is bad for vacuum distillation because it necessitates a higher flash zone temperature and prevents a deeper cut distillation.
The bubble-cap could be replaced by a standard chimney tray having a plurality of risers attached to a plate having holes, with a baffle attached to the top of each riser. Chimney trays are available which provide two 90.degree. direction changes. A first 90.degree. direction change when a stream from the riser contacts the baffle, and a second 90.degree. direction change when the stream exits the chimney. These standard chimneys have a lower pressure drop than bubble-caps; however, they allow significant entrainment.
Liquid entrainment also reduces separation efficiency in other hydrocarbon and non-hydrocarbon services wherein feed entries are flashed. Typical services include produce strippers or towers which are fed a partially vaporized stream.